President Donald Trump will reverse a Environmental Protection Agency (EPA) decision from January to continue implementing Obama-era fuel economy rules, the president announced at the American Center for Mobility in Detroit on Wednesday.

The 2012 standards mandated that, by 2025, automobile companies would have to nearly double the average fuel economy (miles per gallon of gas) of their cars and trucks to nearly 55 miles per gallon; it also provided targeted incentives for automakers develop more innovative technologies — like electric, natural gas, and hybrid vehicles — to reduce greenhouse gas emissions.

Wednesday, however, is expected to be the beginning of the policy’s long unspooling, which will involve more than a year of regulatory review from both the EPA and the Department of Transportation, The New York Times reports. By next April, the administration will propose replacement standards, a White House official told the Times.

As recently as late January, Ford Motor Company CEO Mark Fields –who, along with General Motors Company CEO Mary Barra, met with Trump during his first week as president– suggested to Bloomberg that one million American jobs would be in jeopardy if they weren’t given “some level of flexibility” on the policy. Fields did not provide the source of that figure.

EPA decision on 2012 rules ‘single most important’ in recent history

The fight between the auto industry and the EPA is long-running. As early as 1963, when Congress passed the first iteration of the Clean Air Act — which would later arm the EPA, starting in 1970, with the responsibility to protect and improve national air quality — automakers fiercely resisted the agency’s proposed regulations, alleging, as now, that they could not meet the new standards to reduce harmful emissions.

The most recent rift arose exactly a week before Trump’s inauguration, when the EPA decided to lock in the fuel economy rules — the result of a midterm review mandated as part of the 2012 policy’s implementation. Incensed, auto industry leadership wrote to new EPA head Scott Pruitt after he took office, requesting that the agency withdraw the Jan. 13 decision. They claimed that the analysis that informed its determination was rushed so it could be disseminated just before Trump ‘s inauguration.

“For the auto industry, the Final Determination may be the single most important decision that EPA has made in recent history,” wrote Mitch Bainwol, president and CEO of the Alliance of Automobile Manufacturers (which represents almost 80 percent of auto companies in the U.S, including Ford and General Motors) in a Feb. 21 letter to Pruitt. The EPA’s determination, Bainwol claimed, was “riddled with indefensible assumptions, inadequate analysis, and a failure to engage with contrary evidence.”

Undoing Obama’s environmental legacy

Wednesday’s decision to reopen review of the 2012 fuel rules was celebrated by the auto industry, which applauded the “opportunity” for stakeholders to work with Pruitt and the EPA to reach a “thoughtful,” “coordinated” outcome.

“If [Pruitt] fails to enforce, we’ll have to step into that breach,” said Sierra Club Environmental Law Director Pat Gallagher. “We’ll bring citizen suits against the worst polluters to protect our communities. We will investigate and demand that his conduct as EPA administrator complies with all the ethical rules that govern attorneys and government employees.”